The present invention relates to a refrigeration system, and particularly, though not exclusively, to a refrigeration system forming part of a system for controlling the temperature of an aircraft compartment.
As is known, refrigeration systems basically comprise a compressor, a condenser, an orifice, and an evaporator series-connected to one another to form a closed circuit. The compressor is supplied with refrigerant in the form of low-pressure vapor, which it compresses to form a higher-pressure vapor at the outlet; the vapor is fed into the condenser where it is condensed by heat exchange with a lower-temperature fluid, typically air from outside the environment being temperature controlled; and the liquid from the condenser is fed through the laminating orifice, in which it is expanded, with no change in heat content, to reduce its pressure, and finally to the evaporator. This is a heat exchanger in which the refrigerant exchanges heat with the air inside the environment being temperature controlled; the liquid evaporates and draws heat from the environment; and the vapor from the evaporator is fed to the compressor, i.e. back to the start of the cycle.
The design of a refrigeration system poses two main problems, the relative importance of which depends on the type of application involved, and which are: controlling the temperature of the environment as accurately as possible in response to variations in external conditions; and achieving a high degree of efficiency of the system, i.e. minimizing the mechanical energy required to drive the compressor for a given amount of heat drawn from the environment at a given temperature.
The first problem is solved more or less successfully in various ways.
In less sophisticated applications, such as household refrigerators, a straightforward on-off compressor control is used, which is turned on and off when the temperature inside the refrigerator reaches an upper and lower threshold value, respectively, so that the temperature varies continually between the two threshold values.
In applications involving a variation in the load of the evaporator, a servovalve is sometimes used to control evaporation pressure.
As regards the efficiency problem, some refrigeration systems comprise a so-called economizer for increasing the enthalpy variation at the evaporator for a given capacity of the compressor, and which substantially comprises a second orifice immediately downstream from the condenser for evaporating part of the refrigerant, and a flash tank with an inlet communicating with the second orifice, a liquid-phase outlet connected to the evaporator by the first orifice, and a gaseous-phase outlet connected to an intermediate inlet of the compressor.
The extra mechanical energy required for compressing the fluid withdrawn by the flash tank from the heat exchange in the evaporator is far less than the extra heat drawn from the environment by increasing the enthalpy variation at the evaporator; and the efficiency of the compressor is improved by the inter-refrigeration produced by introducing cold vapor from the flash tank.
To improve both refrigeration efficiency and temperature control, the most logical step would appear to be a combination of the above two solutions (economizer and evaporation pressure control). Unfortunately, however, in certain conditions, operation of the economizer results in disabling of the evaporation pressure control function. That is, the fall in pressure at the inlet of the compressor produced by operation of the evaporation pressure control valve results in a fall in pressure at the intermediate stage of the compressor, so that more vapor is drawn from the flash tank, and the pressure inside the tank is reduced. Being connected to the evaporator, the tank therefore also affects the evaporation pressure, thus making control by the valve pointless.